Mash seam welding process and mash seam welding apparatus

ABSTRACT

Mash seam welding process and apparatus, wherein two plate-like workpieces having different thickness values are fed and guided by a guiding device such that a length W H  of contact of the thick plate-like workpiece with one of two roller electrodes is smaller than a length W M  of contact of the thin plate-like workpiece with the other roller electrode. In this arrangement, the amount of mashing of the thick plate-like workpiece by the roller electrode is made larger than that of the thin plate-like workpiece by the other roller electrode, so that an interface of the two plate-like workpieces is positioned intermediate between the pair of roller electrodes, whereby a weld nugget formed intermediate between the roller electrodes extends across the interface, permitting the weld to have a sufficient strength owing to a nugget ratio, which is a ratio of a width dimension W N  of the weld nugget in the direction of the interface to a width dimension W G  of the interface.

FIELD OF THE ART

This invention relates to a mash seam welding process and a mash seamwelding apparatus for continuously welding together two plate-likeworkpieces by overlapping the workpieces at their edges and applying awelding current between a pair of roller electrodes while theoverlapping edge portions of the two plate-like workpieces are squeezedto be mashed, by and between the pair of roller electrodes.

BACKGROUND TECHNOLOGY

High-speed welding is possible by mash seam welding in which twoplate-like workpieces are continuously welded together at theiroverlapping edge portions while the edge portions are squeezed to bemashed by and between a pair of roller electrodes, with a weldingcurrent being applied between the pair of roller electrodes. Since themash seam welding is suitable for mass production, it is widely employedin production lines for steel plates, automobiles, etc.

In the mash seam welding described above, the two plate-like workpiecesare positioned relative to each other by a clamping or guiding devicethat predetermined portions of the workpieces overlap each other, andthe workpieces are fed so that the overlapping portions are squeezed bythe pair of roller electrodes. Then, the two plate-like workpieces arecontinuously welded with a welding current applied between the rollerelectrodes while the overlapping portions of the workpieces are firmlysqueezed and mashed by the roller electrodes.

In such mash seam welding, a so-called "weld nugget" N is formed basedon a welding current applied to the pair of roller electrodes, such thatthe weld nugget N is located in a central part of the thickness of aportion of two plate-like workpieces 12_(H), 12_(M) squeezed by the pairof roller electrodes, as shown in FIG. 10. The formed weld nugget Nextends across an interface G of the two plate-like workpieces 12_(H),12_(M), so that the weld may be given a predetermined strength (tensilestrength). In FIG. 10, W_(G) represents a width dimension of theinterface G, while W_(N) represents a width dimension of the weld nuggetN in the direction of the interface. The weld strength obtained by themash seam welding depends upon a ratio W_(N) /W_(G), namely, a nuggetratio R_(N), which is the ratio of the width dimension W_(N) of the weldnugget N to the width dimension W_(G) of the interface G. The weldstrength of the mash seal welding increases as this nugget ratio R_(N)increases.

Where two plate-like workpieces having different thickness values arewelded according to a conventional mash seam welding process, theinterface G of the two plate-like workpieces 12_(H), 12_(M) mashed bythe pair of roller electrodes is formed at a position which is offsetfrom the center of the thickness of the mashed portion in the directiontoward the thin plate-like workpiece 12_(M), as indicated in FIG. 11.However, the weld nugget N is formed at a position intermediate betweenthe pair of roller electrodes, that is, at a central part of thethickness of the mashed portion of the two plate-like workpieces 12_(H),12_(M) squeezed between the pair of roller electrodes. Accordingly, itis difficult to form the weld nugget N in alignment with the interface Gof the two plate-like workpieces 12_(H), 12_(M), and is thereforedifficult to obtain the desired weld strength in some cases.

While it is of course possible to increase the nugget ratio R_(N)indicated above by increasing the amount of the welding current tothereby form a large weld nugget N, an increase in the nugget ratioR_(N) leads to easy occurrence of cracking on the surfaces of theplate-like workpieces due to enlargement of the weld nugget N whichundergoes rapid volumetric expansion. This cracking may develop intorupture in a subsequent press-forming process, for example. In view ofthis, the application of the mash seam welding is conventionally limitedto the workpieces whose thickness ratio is not larger than apredetermined upper limit, for example, about 1.5, and the mash seamwelding is not conventionally practiced where the thickness ratio of theworkpieces exceeds the upper limit.

On the other hand, there has been proposed a seam welding process inwhich a shim is superposed on a thinner one of two plate-like workpiecesso that the interface G of the two plate-like workpieces is locatedintermediate between a pair of roller electrodes, as disclosed inJP-A-63-63575. This seam welding process requires the shim as well asthe two plate-like workpieces to be clamped by a clamping device adaptedto position the two plate-like workpieces relative to each other.Therefore, this process has a drawback that the operation efficiency isinevitably lowered. Further, where the two plate-like workpieces arepositioned relative to each other by using guide rollers while theworkpieces are fed toward the roller electrodes, there is a drawbackthat the above seam welding process using the shim cannot be practiced.

The present invention was made in the light of the above backgroundsituation. It is therefore an object of the present invention to providea mash seam welding process and a mash seam welding apparatus, whichpermit a sufficient weld strength even where two plate-like workpieceshave different thicknesses.

DISCLOSURE OF THE INVENTION

The above object may be achieved according to the principle of thepresent invention, which provides a mash seam welding processcontinuously welding a first plate-like workpiece and a secondplate-like workpiece which has a smaller thickness than the firstplate-like workpiece and which overlaps the first plate-like workpiece,by applying a welding current between a pair of roller electrodesdisposed rotatably about respective axes thereof, to thereby form a weldnugget at an interface of the two plate-like workpieces while theoverlapping portions are squeezed by the pair of roller electrodes,wherein an amount of mashing of the first plate-like workpiece by one ofthe pair of roller electrodes is made larger than that of the secondplate-like workpiece by the other of the pair of roller electrodes.

In the present process in which the amount of mashing of the first thickplate-like workpieces by the first plate-like workpieces is larger thanthat of the second thin plate-like workpiece, the position of theinterface of the two plate-like workpieces is shifted to a positionintermediate between the pair of roller electrodes, so that the weldnugget formed at the position intermediate between the pair of rollerelectrodes extends across the above-indicated interface, permitting asufficient weld strength.

Preferably, a surface area of contact of the above-indicated one rollerelectrode with the first plate-like workpiece is made smaller than asurface area of contact of the other roller electrode with the secondplate-like workpiece. This arrangement is advantageous in that theamount of mashing of the first thick plate-like workpiece is made largerthan that of the second thin plate-like workpiece, when the pair ofplate-like workpieces are squeezed and mashed by the pair of rollerelectrodes.

It is also preferable that wherein a length of contact of theabove-indicated one roller electrode with the first plate-like workpiecein an axial direction of the pair of roller electrodes is made smallerthan that of the other roller electrode with the second plate-likeworkpiece in the axial direction. This arrangement is also advantageousin that the amount of mashing of the first thick plate-like workpiece ismade larger than that of the second thin plate-like workpiece, when thepair of plate-like workpieces are squeezed and mashed by the pair ofroller electrodes.

It is also preferable that the above-indicated one roller electrodelocated on the side of the first plate-like workpiece has a smallerdiameter than the other roller electrode located on the side of thesecond plate-like workpiece. This arrangement is advantageous in thatthe amount of mashing of the first thick plate-like workpiece is madelarger than that of the second thin plate-like workpiece, even thoughthe lengths of contact of the pair of roller electrodes with the firstand second plate-like workpieces are the same.

It is also preferable that the pair of plate-like workpieces are fed ata speed of 20 m/min. or higher while the plate-like workpieces aresqueezed by the above-indicated pair of roller electrodes. In thisarrangement in which the welding speed is increased, the contactresistance of the pair of roller electrodes and the two plate-likeworkpieces is made higher than the specific resistance of the plate-likeworkpieces, so that the weld nugget formed within the two plate-likeworkpieces is formed along the interface, with a result of an increasein the nugget ratio, permitting the weld to have a sufficient strengtheven where the pair of roller electrodes have different surface areas ofcontact with the two plate-like workpieces having different thicknessvalues.

The above object may also be achieved according to another aspect of theinvention, which provides a mash seam welding process of continuouslywelding a first plate-like workpiece and a second plate-like workpiecewhich has a smaller thickness than the first plate-like workpiece andwhich overlaps the first plate-like workpiece, while overlappingportions of the two plate-like workpieces are squeezed a pair of rollerelectrodes disposed rotatably about respective two parallel axesthereof, the process comprising: a welding step of continuously weldingthe two plate-like workpieces by applying a welding current between apair of roller electrodes, to thereby form a weld nugget at an interfaceof the two plate-like workpieces, while the two plate-like workpiecesare squeezed by the pair of roller electrodes; and a preliminarytreatment step of reducing the thickness of a welding portion of thefirst plate-like workpiece, prior to the welding step. In thisarrangement, the thickness of the welding portion of the first thickplate-like workpiece is first reduced, so as to reduce the thicknessdifference with respect to the welding portion of the second thinplate-like workpiece, so that the weld nugget formed at the positionintermediate between the pair of roller electrodes extends across theinterface of the two plate-like workpieces, permitting the weld to havea sufficient strength.

The above process of the present invention may be suitably practiced bya mash seam welding apparatus for continuously welding a firstplate-like workpiece and a second plate-like workpiece which has asmaller thickness than the first plate-like workpiece and which overlapsthe first plate-like workpiece, by applying a welding current between apair of roller electrodes disposed rotatably about respective axesthereof, to thereby form a weld nugget at an interface of the twoplate-like workpieces while the overlapping portions are squeezed by thepair of roller electrodes, the apparatus comprising: (a) a pair ofroller electrodes disposed rotatably about respective two parallel axesthereof and biased toward each other, the pair of roller electrodeshaving a same diameter; and (b) a guiding device for guiding the twoplate-like workpieces such that the two plate-like workpieces overlapeach other with a predetermined lap amount, while the two plate-likeworkpieces are fed toward the pair of roller electrodes, so that the twoplate-like workpieces are squeezed by the pair of roller electrodes suchthat a surface area of contact of one of the pair of roller electrodeswith the first plate-like workpiece is smaller than a surface area ofcontact of the other of the pair of roller electrodes with the secondplate-like workpiece.

In the present apparatus wherein the surface area of contact of one ofthe above-indicated pair of roller electrodes with the first plate-likeworkpiece is made smaller than that of the other roller electrode withthe second plate-like workpiece, the amount of mashing of the firstthick plate-like workpiece is made larger than that of the second thinplate-like workpiece when the pair of plate-like workpieces are squeezedand mashed by the pair of roller electrodes. Accordingly, the positionof the interface of the two plate-like workpieces is shifted to aposition intermediate between the pair of roller electrodes, so that theweld nugget formed at the position intermediate between the pair ofroller electrodes extends across the above-indicated interface,permitting a sufficient weld strength.

Preferably, the guiding device guides the two plate-like workpieces suchthat a length of contact of the one roller electrode with the firstplate-like workpiece in an axial direction of the pair of rollerelectrodes is smaller than that of the other roller electrode with thesecond plate-like workpiece in the axial direction. This arrangement isadvantageous in that the amount of mashing of the first thick plate-likeworkpiece is made larger than that of the second thin plate-likeworkpiece when the pair of plate-like workpieces are squeezed and mashedby the pair of roller electrodes.

In one arrangement of the above preferred form of the apparatus, thepair of roller electrodes have respective outer circumferential contactsurfaces and are disposed such that widthwise centerlines of the outercircumferential contact surfaces are substantially aligned with eachother, and the guiding device guides the pair of plate-like workpiecessuch that an edge of the first plate-like workpiece is located between awidthwise end of the outer circumferential contact surface of the oneroller electrode on the side of the first plate-like workpiece and thewidthwise centerline of the outer circumferential contact surface of theone roller electrode, while an edge of the second plate-like workpieceis located at an widthwise end of the outer circumferential contactsurface of the other roller electrode on the side of the firstplate-like workpiece.

In another arrangement of the above preferred form of the apparatus, thepair of roller electrodes have respective outer circumferential contactsurfaces and are disposed such that widthwise centerlines of the outercircumferential contact surfaces are offset from each other by apredetermined distance in an axial direction thereof, and wherein theguiding device guides the two plate-like workpieces such that acenterline of the overlapping edge portions of the two plate-likeworkpieces is substantially aligned with the widthwise centerline of theouter circumferential contact surface of the one of the pair of rollerelectrodes which is located on the side of the plate-like workpiecehaving a smaller thickness.

In a further arrangement of the above preferred form of the apparatus,the roller electrode which is located on the side of the plate-likeworkpiece having a larger thickness has an outer circumferential contactsurface having a width dimension smaller than an outer circumferentialcontact surface of the other of the pair of roller electrodes which islocated on the side of the plate-like workpiece having a smallerthickness, the pair of roller electrodes being disposed such thatwidthwise centerlines of the outer circumferential contact surfaces aresubstantially aligned with each other, and wherein the guiding deviceguides the pair of plate-like workpieces such that a centerline of theoverlapping edge portions of the two plate-like workpieces issubstantially aligned with the widthwise centerlines of the outercircumferential contact surfaces of the roller electrode.

It is also preferable that the one of the pair of roller electrodeslocated on the side of the first plate-like workpiece has a smallerdiameter than the other roller electrode located on the side of thesecond plate-like workpiece. This arrangement is advantageous in thatthe amount of mashing of the first thick plate-like workpiece is madelarger than that of the second thin plate-like workpiece, even where theguiding device is adapted to guide the pair of plate-like workpiecessuch that the lengths of contact of the pair of roller electrodes withthe first and second plate-like workpieces in the axial direction areequal to each other.

It is also preferable that the apparatus further comprises a feedingdevice for feeding the pair of plate-like workpieces at a speed of 20m/min. or higher while the pair of plate-like workpieces are squeezed bythe pair of roller electrodes. In this arrangement, the feeding devicefor feeding the plate-like workpieces permits a high welding speed, andcauses a higher resistance of contact of the pair of roller electrodeswith the two plate-like workpieces than the specific resistance of theplate-like workpieces, whereby the nugget ratio of the weld nuggetformed along the interface of the two plate-like workpieces isincreased, advantageously permitting a sufficient weld strength.

The above object may also be achieved according to a further aspect ofthe invention, which provides a mash seam welding apparatus forcontinuously welding a first plate-like workpiece and a secondplate-like workpiece which has a smaller thickness than the firstplate-like workpiece and which overlaps the first plate-like workpiece,while the overlapping portions of the two plate-like workpieces aresqueezed by a pair of roller electrodes disposed rotatably aboutrespective two parallel axes thereof, characterized by comprising: awork feeding device for feeding the two plate-like workpieces in onedirection; a welding portion for continuously welding the two plate-likeworkpieces by applying a welding current between a pair of rollerelectrodes, to thereby form a weld nugget at an interface of the twoplate-like workpieces, while the two plate-like workpieces are fed bythe work feeding device and squeezed by the pair of roller electrodes;and a preliminary treatment device disposed upstream of the weldingportion, for reducing the thickness of a welding portion of the firstplate-like workpiece. In this arrangement wherein the thickness of thewelding portion of the first thick plate-like workpiece is reduced priorto the welding by the pair of roller electrodes, the thicknessdifference of the welding portion of the first thick plate-likeworkpiece with respect to that of the second thin plate-like workpieceis reduced, so that the weld nugget formed intermediate between the pairof roller electrodes extends across the interface of the two plate-likeworkpieces, advantageously permitting a sufficient weld strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing a manner of feedingplate-like workpieces to explain a mash seam welding process of thepresent invention;

FIG. 2 is a plan view explaining an arrangement of guide rollers andretainer rollers used for feeding the plate-like workpieces, to explainthe mash seam welding process according to the embodiment of FIG. 1;

FIG. 3 is a view showing portions of guide members by which theplate-like workpieces are slidably guided, to explain a workpieceguiding step in the embodiment of FIG. 1;

FIG. 4 is a view explaining a positional relationship between a pair ofroller electrodes and the edge portions of the pair of plate-likeworkpieces pinched by the roller electrodes in a welding step;

FIG. 5 is a view explaining the position of a weld nugget formed in thewelding step in the embodiment of FIG. 1;

FIG. 6 is view in cross section showing the guide rollers and theirvicinity of a seam welding apparatus suitable for practicing the seamwelding process of FIG. 1, taken in a plane perpendicular to a feedingdirection of the plate-like workpieces;

FIG. 7 is a view showing in enlargement a portion of the seam weldingapparatus of FIG. 6;

FIG. 8 is a view showing the roller electrodes and their vicinity of theseam welding apparatus of FIG. 6, taken in the plane perpendicular tothe feeding direction of the plate-like workpieces;

FIG. 9 is a partly cut-away view explaining the roller electrodes andtheir vicinity of the seam welding apparatus of FIG. 6;

FIG. 10 is a view for explaining a definition of a nugget ratio;

FIG. 11 is a view explaining the position of a weld nugget generated ina conventional mash seam welding process;

FIG. 12 is a view corresponding to that of FIG. 4, showing anotherembodiment of this invention;

FIG. 13 is a view corresponding to that of FIG. 4, showing anotherembodiment of this invention;

FIG. 14 is a side elevational view explaining diameters of a pair ofroller electrodes in another embodiment of this invention;

FIG. 15 is a view corresponding to that of FIG. 4, showing anotherembodiment of this invention;

FIG. 16 is a view explaining the position at which a weld nugget isformed in the embodiment of FIG. 15;

FIG. 17 is a view and a table indicating results of experimentsconducted to confirm a range of welding speed in which the embodiment ofFIG. 15 is effective;

FIG. 18 is a view explaining the position at which a weld nugget isformed when the embodiment of FIGS. 1-9 and the embodiment of FIG. 15are combined; and

FIG. 19 is a perspective view explaining a preliminary treatment or apreliminary treatment device in another embodiment of this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

There will be described embodiments of this invention in detailreferring to the drawings. FIG. 1 through FIG. 5 are views explaining aseam welding process to be practiced according to this invention, whileFIG. 6 through FIG. 9 are views explaining a seam welding apparatus 10adapted to practice the seam welding process.

In FIGS. 1 and 2, there are shown a pair of left and right plate-likeworkpieces 12_(H), 12_(M) which are separate plates such as zinc steelplates having given shapes. The plate-like workpiece 12_(H) has a largerthickness than the plate-like workpiece 12_(M). These plate-likeworkpieces 12_(H), 12_(M) are fed in a predetermined feeding direction Bby a plurality of sets of guide rollers 14 (Feeding Step), while the twoplate-like workpieces 12_(H), 12_(M) are guided and positioned relativeto each other by a pair of upper and lower plate-like guide members18_(U), 18_(S) in sliding contact with guide surfaces 24_(U), 24_(S)such that edge portions of the plate-like workpieces 12_(H), 12_(M)overlap each other with a predetermined lap amount (lap width) S(Guiding Step). The thus positioned two plate-like workpieces 12_(H),12_(M) are squeezed under pressure at their overlapping edge portions,by a pair of upper and lower roller electrodes 20_(U), 20_(S). In thiscondition, an electric current supplied from a power source is appliedbetween the pair of roller electrodes 20_(U), 20_(S), whereby the twoplate-like workpieces 12_(H), 12_(M) are welded together at theiroverlapping edge portions, along a welding centerline A which isparallel to the feeding direction B and which passes a center of the lapwidth S of the overlapping edge portions (Welding Step). A set ofretainer rollers 16 are disposed on opposite sides of the rollerelectrodes 20_(U), 20_(S), for holding the pair of plate-like workpieces12_(H), 12_(M) under welding, with a predetermined squeezing force inthe direction of thickness of the workpieces, so as to prevent movementof the workpieces away from each other (Holding Step).

Each of the above-indicated sets of guide rollers 14 consists of a pairof upper and lower guide rollers 14_(HU), 14_(HS) disposed to pinch theplate-like workpiece 12_(H) with a predetermined pinching force in thedirection of thickness of the workpiece, and another pair of upper andlower guide rollers 14_(MU), 14_(MS) disposed to pinch the otherplate-like workpiece 12_(M) with a predetermined pinching force in thedirection of thickness of the workpieces. The sets of guide rollers 14are arranged in the feeding direction B, and include at least one set ofguide rollers 14 which are positively driven. Each guide roller 14_(HU),14_(HS), 14_(MU), 14_(MS) of each set is supported rotatably about ahorizontal axis C1 which lies in a plane inclined a suitable angle θwith respect to a plane perpendicular to the welding centerline A andwhich is located above or below the welding centerline A. Each pair ofupper and lower guide rollers 14_(HU) and 14_(HS), or 14_(MU) and14_(MS) is given a suitable pinching force by a cylinder device 56_(H),56_(M) described below, for example, such that the pinching force actson the upper and lower guide rollers in a direction toward each other.The pinching force is determined so as to generate a friction forceenough to permit the upper and lower rollers to generate a forceconsisting of a component for feeding the plate-like workpiece 12_(H),12_(M) in the feeding direction B and component for moving the workpiecein the direction toward the guide members 18_(U), 18_(S).

The above-indicated set of retainer rollers 16 consists of a pair ofupper and lower retainer rollers 16_(HU), 16_(HS) which are disposed onone side of the roller electrodes 20_(U), 20_(S) to hold the plate-likeworkpiece 12_(H) with a predetermined squeezing force in the directionof thickness of the workpiece, and another pair of upper and lowerretainer rollers 16_(MU), 16_(MS) disposed on the other side of theroller electrodes 20_(U), 20_(S) to hold the other plate-like workpieces12_(M) with a predetermined squeezing force in the direction ofthickness of the workpiece. Thus, the two pairs of retainer rollers 16are disposed such that the roller electrodes 20_(U), 20_(S) areinterposed between the retainer rollers 16. Each pair of upper and lowerretainer rollers 16_(HU) and 16_(HS), or 16_(MU) and 16_(MS) is given acomparatively large squeezing force by a cylinder device similar to thecylinder device 56_(H), 56_(M) described below, for example, such thatthe squeezing force acts on the upper and lower retainer rollers in adirection toward each other. The squeezing force is determined so as toprevent the edge portions of the pair of plate-like workpieces 12H, 12Munder welding from moving away from each other. Each retainer roller16_(HU), 16_(HS), 16_(MU), 16_(MS) is supported rotatably about ahorizontal axis C2 which lies in a plane perpendicular to the weldingcenterline A and which is located above or below the welding centerlineA. The plate-like workpieces 12_(H), 12_(M) under feeding, namely, underwelding are retained by the above-indicated squeezing force to preventthe movements of their edge portions away from each other (HoldingStep).

Each of the above-indicated pair of roller electrodes 20_(U), 20_(S) issupported rotatably about a horizontal axis C3 which lies in a planeperpendicular to the welding centerline A and which is located above orbelow the welding centerline A. The pair of roller electrodes 20_(U),20_(S) is given a comparatively large squeezing force by a cylinderdevice by a cylinder device 80 described below, for example, such thatthe squeezing force acts on the roller electrodes in a direction towardeach other. The squeezing force is determined so as to permit theoverlapping edge portions of the plate-like workpieces 12_(H), 12_(M) tobe mashed by an amount enough to reduce the overall lap thickness to theapproximate thickness of one of the workpieces 12_(H), 12_(M). Thesqueezing portion of the roller electrodes 20_(U), 20_(S) is indicatedat P in FIG. 2.

As shown in detail in FIG. 3, the above-indicated plate-like guidemembers 18U, 18S are disposed with a predetermined amount of gap leftbetween mutually opposed end faces thereof, so as to extend along thewelding centerline A and between the pair of upper and lower guiderollers 14_(HU), 14_(HS) for holding the one plate-like workpiece12_(H), and the pair of upper lower guide rollers 14MU, 14MS for holdingthe other plate-like workpiece 12M, such that the guide members 18U, 18Sare spaced apart by a predetermined small distance from the pair ofroller electrodes 20_(U), 20_(S). The upper guide member 18_(U) has aprojecting rail 26_(U) formed at a portion of its end face on the sideof the plate-like workpiece 12_(M). The projecting rail 26_(U) has aguide surface 24_(U) for sliding contact with the end face of theplate-like workpiece 12_(H). The lower guide member 18_(S) has aprojecting rail 26_(S) formed at a portion of its end face on the sideof the plate-like workpiece 12_(H). The projecting rail 26_(S) has aguide surface 24_(S) for sliding contact with the end face of theplate-like workpiece 12_(M).

As shown in FIG. 2, the rotation axis C1 of at least one set (all setsin this embodiment) of the guide rollers 14 disposed on the left andright sides of the guide members 18_(U), 18_(S) is inclined apredetermined small angle θ (1°-3° in this embodiment) with respect tothe line perpendicular to the welding centerline A, so that theplate-like workpieces 12_(H), 12_(M) are driven or guided by the guideroller 14 in a direction (in the direction of the driving force F_(R)described below) which is inclined inwardly by the predetermined angle θwith respect to the feeding direction B.

When the plate-like workpiece 12_(M) is in abutting contact with theguide surface 24_(S), for example, the plate-like workpiece 12_(M) isfed while it is pressed toward the guide surface 24_(S). Similarly, theplate-like workpiece 12_(H) is fed while it is pressed toward the guidesurface 24_(U). Thus, the pair of plate-like workpieces 12_(H), 12_(M)are positioned relative to each other with their edge portionsoverlapping over the predetermined lap width S, before the workpiecesare squeezed by the upper and lower roller electrodes 20_(U), 20_(S). Inthis arrangement, the pair of plate-like workpieces 12_(H), 12_(M) areautomatically positioned relative to each other during feeding thereofthrough a feeding path, even if the workpieces do not have thepredetermined relative position when they are introduced into thefeeding path. The above-indicated guide members 18_(U), 18_(S) areadapted to guide the plate-like workpieces 12_(H), 12_(M) such that theedge portions of the plate-like workpieces 12_(H), 12_(M) squeezedbetween the roller electrodes 20_(U), 20_(S) are positioned as indicatedin FIG. 4. Namely, the guide members 18_(U), 18_(S) are disposed at theappropriate positions for positioning the workpieces as indicated inFIG. 4. In the guiding step in the present embodiment, the pair ofplate-like workpieces 12_(H), 12_(M) are guided and positioned relativeto each other by the above-indicated guide members 18_(U), 18_(S) andthe guide rollers 14 disposed on the left and right sides of the guidemembers 18_(U), 18_(S), so that the edge portions of these plate-likeworkpieces 12_(H), 12_(M) may be squeezed by the roller electrodes20_(U), 20_(S), as indicated in FIG. 4. In the present embodiment, theabove guide members 18_(U), 18_(S) and the guide rollers 14 disposed onthe left and right sides of the guide members 18_(U), 18_(S) function asa guiding device.

FIG. 4 shows the overlapping edge portions of the thick plate-likeworkpiece 12_(H) and the plate-like workpiece 12_(M) having a thicknesssmaller than the plate-like workpiece 12_(H), which overlapping edgeportions are positioned between the roller electrodes 20_(U), 20_(S)which have respective outer circumferential contact surfaces 28_(U),28_(S) and which are disposed such that widthwise centerlines CL_(U),CL_(S) of the outer circumferential contact surfaces 28_(U), 28_(S) aresubstantially aligned with each other. The edge of the thick plate-likeworkpiece 12_(H) is located between the widthwise end of the outercircumferential contact surface 28U of the one roller electrode 20_(U)on the side of the plate-like workpiece 12_(H) and the centerlineCL_(U), while the edge of the thin plate-like workpiece 12_(M) islocated at the widthwise end of the outer circumferential contactsurface 28_(S) of the other roller electrode 20_(S) on the side of theplate-like workpiece 12_(H). That is, a length W_(H) of contact of theone roller electrode 20_(U) with the plate-like workpiece 12_(H) issmaller than a length W_(M) of contact of the other roller electrode20_(S) with the plate-like workpiece 12_(M). When the edge portions ofthe plate-like workpiece 12_(H) and the plate-like workpiece 12_(S)whose thickness is smaller than the plate-like workpiece 12_(H) aresqueezed between the roller electrodes 20_(U), 20_(HS) while these edgeportions overlap each other as shown in FIG. 4, the amount of mashing ofthe thick plate-like workpiece 12_(H) is larger than that of the thinplate-like workpiece 12_(M), since the contact surface area of theroller electrode 20_(U) with respect to the plate-like workpiece 12_(H)is smaller than the contact surface area of the roller electrode 20_(S)with respect to the plate-like workpiece 12_(S). Consequently, theinterface G of the two plate-like workpieces 12_(H), 12_(M) is formed ata position intermediate between the pair of roller electrodes 20_(U),20_(S), whereby the weld nugget N formed is more likely to extend acrossthe above-indicated interface G. Accordingly, the present arrangementresults in an increase in the nugget ratio R_(N) which is the ratioW_(N) /W_(G) (ratio of the width W_(N) of the weld nugget N in thedirection of the interface G to the width dimension W_(G) of theinterface G), so that the weld can be given a sufficient strength.

The mash seam welding apparatus 10 for practicing the mash seam weldingprocess explained above is constructed as shown in FIGS. 6-9. FIG. 6 isa cross sectional view perpendicular to the feeding direction, showingthe guide rollers 14 and their vicinity of the mash seam weldingapparatus 10. FIG. 7 is a view showing in enlargement the guide rollers14 and their vicinity. FIG. 8 is a view explaining an arrangement of theroller electrodes 20_(U), 20_(S) and their vicinity, while FIG. 9 is apartly cut-away view explaining the support structure for the rollerelectrode 20_(U).

Since the guide rollers 14 and the retainer rollers 16 are basicallyidentical in construction to each other, the construction of the guiderollers 14 and the support structure thereof will be described by way ofexample.

Referring to FIGS. 6 and 7, a base frame 50 extends in the weldingdirection B, and a pair of posts 52_(H), 52_(M) are disposed upright onthe base frame 50. On the upper end faces of the posts 52_(H), 52_(M),there are provided the guide rollers 14_(HS), 14_(MS) via base blocks53_(H), 53_(M). A top frame 55 is fixed to the base frame 50 through anouter frame 54. To the top frame 55, there are fixed a pair of left andright cylinder devices 56_(H), 56_(M) for giving the pinching forces tothe pair of upper and lower guide rollers 14_(HU), 14_(HS) and the pairof upper and lower guide rollers 14_(MH), 14_(MS), respectively. Movablerods 58_(H), 58_(M) extend downward from the cylinder devices 56_(H),56_(M), such that the movable rods 58_(H), 58_(S) are movable. To thelower ends of the movable rods 58_(H), 58_(M), there are fixed movableplates 60_(H), 60_(M) on which the guide rollers 14_(HU), 14_(MU).

The set of guide rollers 14_(HS), 14_(MS), 14_(HU), 14_(MU) are providedwith respective integrally formed shafts 66 which are supported byrespective shaft cases 62_(HS), 62_(MS), 62_(HU), 64_(MU) throughbearings 64. The above-indicated shaft cases 62_(HS), 62_(MS) are fixedto the upper end faces of the posts 52_(H), 52_(M), while theabove-indicated shaft cases 62_(HU), 62_(MU) are fixed to the movableplates 60_(H), 60_(M). The guide rollers 14_(HS), 14_(MS) are rotatableabout the axis C1 located below the welding centerline A while the guiderollers 14_(HU), 14_(MU) are rotatable about the axis C, located abovethe welding centerline A. As shown in FIG. 3, the guide members 18_(U),18_(S) are fixed to the cylinder device 56_(H) and the post 52_(M),respectively, such that the guide members 18_(U), 18_(S) are disposedwith the predetermined amount of gap left between their mutually opposedend faces, and extend between the pair of upper and lower guide rollers14_(HU), 14_(HS) for holding the one plate-like workpiece 12_(H), andthe pair of upper lower guide rollers 14_(MU), 14_(MS) for holding theother plate-like workpiece 12_(M), such that the welding centerline A isaligned with the widthwise centers of the opposed end faces of the guidemembers 18U, 18S.

Each of the shafts 66 of the lower guide rollers 14_(HS), 14_(MS) isconnected to a drive motor 76 through a pair of joints 68, 70, anintermediate shaft 72, and a chain 74. The intermediate shaft 72 isprovided for each of the left and right guide rollers 14_(HS), 14_(MS)of each set of guide rollers 14, and these intermediate shafts 72 areconnected to each other by chains and to the drive motor 76, forsimultaneous rotation at the same speed. In this arrangement, the lowerrollers 14_(HS), 14_(MS) of the guide rollers 16 are rotated by thedrive motor 76 to feed the pair of plate-like workpieces 14HS, 14MS. Inthe present embodiment, the drive motors 76 function as a driving devicefor rotating the guide rollers 14. Similar drive motors are used as adriving device for rotating the retainer rollers 16.

The pairs of upper and lower guide rollers 14_(HU), 14_(HS), 14_(MU),14_(MS) are steel rollers whose outer circumferential surfaces arecovered by an elastic material such as urethane rubber. The pair ofguide rollers 14_(HU), 14_(HS) and the pair of guide rollers 14_(MU),14_(MS) are squeezed toward each other by the respective cylinderdevices 56_(H), 56_(M) which function as a squeezing force applyingdevice, so that the squeezing forces acting on the guide rollers permitthe generation of friction forces enough to generate the component forfeeding the pair of plate-like workpieces 12_(H), 12_(M) and thecomponent for forcing the workpieces toward the guide members 18_(U),18_(S).

The pair of upper and lower retainer rollers 16_(HU), 16_(HS) and thepair of upper and lower retainer rollers 16_(MU), 16_(MS) whichconstitute one set of retainer rollers 16 are constructed similarly tothe guide rollers 14_(HU), 14_(HS), 16_(MU), 14_(MS), as shown in FIG.8. The retainer rollers 16 are rotatable about the respective rotationaxes C2 described above, and are rotated by the drive motors 76 at thesame peripheral speed as the guide rollers 14_(HS), 14_(MS). The pair ofupper and lower retainer rollers 16_(HU), 16_(HS) and the pair of upperand lower retainer rollers 16_(MU), 17_(MS) are steel rollers whoseouter circumferential surfaces are knurled to increase the generatedfriction forces in their axial direction, with respect to the plate-likeworkpieces 12_(H), 12_(M). The retainer rollers 16_(HS), 16_(HU) and theretainer rollers 16_(MS), 16_(MU) are squeezed toward each other bycylinder devices similar to the cylinder devices 56_(H), 56_(M), whichalso function as a squeezing force applying device. The squeezing forcesgiven to the retainer rollers 16 are comparatively large permitting thegeneration of friction forces on their outer circumferential surfaces,so as to prevent the movements of the edge portions of the pair ofplate-like workpieces 12_(H), 12_(M) under welding in the directionsaway from each other.

Auxiliary roller sets 130_(HU), 130_(HS), 130_(MU), 130_(MS) areinterposed between the roller electrodes 20_(U), 20_(S) and the pair ofretainer rollers 16_(HU), 16_(HS), and between the roller electrodes20_(U), 20_(S) and the pair of retainer rollers 16_(MU), 16_(MS), in thedirection perpendicular to the feeding direction B, that is, in theaxial direction of the roller electrodes. These auxiliary roller sets130_(HU), 130_(HS), 130_(MU), 130_(MS) are located so as to press theportions of the workpieces at which the convex deformation F is expectedto take place. Described more specifically, the auxiliary roller sets130_(HS) and 130_(MS) located below the plate-like workpieces 12_(H),12_(M) are rotatably supported by the end portions of brackets 132_(HS)and 132_(MS) fixed to the base blocks 53_(H), 53_(M), while theauxiliary roller sets 130_(HU) and 130_(MU) located above the plate-likeworkpieces 12_(H), 12_(M) are rotatably supported by the end portions ofbrackets 132_(HU) and 132_(MU) fixed to the movable plates 60_(H),60_(M). The above auxiliary roller sets 130_(HU), 130_(HS), 130_(MU),130_(MS) are rotatable about axes C4 which are parallel to the rotationaxes C3 of the roller electrodes 20_(U), 20_(S) and the axes C2 of thepair of retainer rollers 16_(HU), 16_(HS). The plate-like workpieces12_(H), 12_(M) are pressed with predetermined squeezing forces at theirportions laterally adjacent to the roller electrodes 20_(U), 20_(M), bythe upper and lower auxiliary roller sets 130_(HU), 130_(HS) and theupper and lower auxiliary roller sets 130_(MU), 130_(MS).

The pair of roller electrodes 20_(U), 20_(S) have the same diameter,width and construction, as shown in FIG. 8, and only the upper rollerelectrode 20_(U) will be explained by reference to FIG. 9, which is apartly cut-away view showing the upper roller electrode 20_(U) and thesupport structure of this electrode. As shown in FIG. 9, a cylinderdevice 80_(U) is fixed to the above-described top frame 55, for givingthe roller electrodes 20_(U), 20_(S) a squeezing force. The cylinderdevice 80_(U) is provided with movable rods 82_(U) extending downwardsuch that the movable rods 82_(U) are movable. A movable plate 84 isfixed to the ends of the movable rods 82U, and a support block 86 forrotatably supporting the roller electrode 20_(U) is fixed to the movableplate 84.

The roller electrode 20_(U) comprised of a rotary shaft 90_(U) with aflange portion 88_(U) integrally formed at an axially intermediate partthereof, and a circular electrode disk 96_(U) fastened by a screw 94_(U)to the flange portion 88_(U) together with a pressure plate 92_(U). Therotary shaft 90_(U) is fitted at its opposite end portions through powersupply bushings 98_(U), in a through-hole 100_(U) formed through theabove-indicated support block 86_(U), whereby the rotary shaft 90_(U) isrotatably supported by the support block 86_(U). The above electrodedisk 96_(U) is a highly electrically conductive body formed of ametallic material such as chromium steel, beryllium copper alloy orchromium zirconium copper alloy, which is highly durable with a smallamount of wear or damage even in the presence of a comparatively largeelectric current applied thereto while the metallic material is incontact with the plate-like workpieces 12_(H), 12_(M). The rotary shaft90_(U), pressure plate 92_(U), power supply bushings 98_(U) and supportblocks 86_(U) are highly electrically conductive bodies formed of ametallic material such as copper alloy or aluminum alloy. The electriccurrent is supplied from the above-indicated power source 22 to theelectrode disk 96_(U) through electric wires not shown, and through thepower supply bushings 98, rotary shaft 90_(U), etc.

A fluid passage 102_(U) is formed through the above-indicated rotaryshaft 90_(U) such that the fluid passage 102_(U) is open in the oppositeend faces of the rotary shaft 90_(U) and has an intermediate portion inthe form of a circular recess formed in the surface of the flangeportion 88_(U) which contacts the electrode disk 96_(U). A pair of hosejoints 108_(U), 110_(U) are fixed to the end faces of the rotary shaft90_(U), for connecting the fluid passage 102_(U) to coolant fluid hoses104_(U), 106_(U) while permitting the rotary shaft 90U to rotate.

In the mash seam welding apparatus 10 constructed as described above,the plurality of sets of guide rollers 14 and the one set of retainerrollers 16 are continuously rotated by the drive motors 76, so that thepair of plate-like workpieces 12_(H), 12_(M) are fed and positionedrelative to each other by the guide rollers 14 and the guide members18_(U), 18_(S) such that the edge portions of the workpieces overlapeach other over the predetermined lap width S. Then, the workpieces aresqueezed and mashed by the pair of roller electrodes 20_(U), 20_(S), anda welding current is simultaneously applied between the rollerelectrodes 20_(U), 20_(S), whereby the workpieces are welded at theiredge portions as shown in FIG. 5, for example, as described above.

In the present embodiment, the pair of plate-like workpieces 12_(H),12_(M) are guided in the guiding step by the guiding device (guiderollers 14 and guide members 18_(U), 18_(S)), in a manner as shown inFIG. 4, so that the workpieces are introduced between the pair of rollerelectrodes 20_(U), 20_(S). Namely, the pair of plate-like workpieces12_(H), 12_(M) are guided during feeding thereof such that the lengthW_(H) of contact of the one roller electrode 20_(U) with the plate-likeworkpiece 12_(H) is smaller than the length W_(M) of contact of theother roller electrode 20_(S) with the plate-like workpiece 12_(M), inother words, such that the area of contact of the thick plate-likeworkpiece 12_(H) with the roller electrode 20_(U) is smaller than thearea of contact of the thin plate-like workpiece 12_(M) with the rollerelectrode 20_(S). In this arrangement, the amount of mashing of theplate-like workpiece 12_(H) by the one roller electrode 20_(U) is largerthan that of the plate-like workpiece 12_(M) by the other rollerelectrode 20_(S). Consequently, the interface G of the two plate-likeworkpieces 12_(H), 12_(M) is formed at a position intermediate betweenthe pair of roller electrodes 20_(U), 20_(S), whereby the weld nugget Nformed is more likely to extend across the above-indicated interface G,as shown in FIG. 5. Accordingly, the present arrangement results in anincrease in the nugget ratio R_(N) which is the ratio W_(N) /W_(G)(ratio of the width W_(N) of the weld nugget N in the direction of theinterface G to the width dimension W_(G) of the interface G), so thatthe weld can be given a sufficient strength.

The present inventors conducted an experiment in which a mash seamwelding operation was performed in the following condition, for weldingthe plate-like workpiece 12_(H) in the form of a 1.4 mm-thick zinc steelplate and the plate-like workpiece 12_(M) in the form of a 0.7 mm-thickzinc steel plate: lap width S of the overlapping edge portions of theworkpieces 12_(H), 12_(M) =1.0 mm; offset distance L₁ of the weldingcenterline A from the widthwise centerlines CL_(U), CL_(S) of the outercircumferential contact surfaces 28_(U), 28_(S) of the roller electrodes20_(U), 20_(S) =2.0 mm; squeezing force acting on the upper and lowerroller electrodes 20_(U), 20_(S) =9800N (=1000 kgf); welding current=27kA; and welding speed =20 m/min. In the experiment, the nugget ratioR_(N) was 50%, and the weld obtained had a sufficient strength.

It is noted that a guiding device in a conventional mash seam weldingapparatus is adapted to guide the plate-like workpieces 12_(H), 12_(M)such that the surface area of contact of the roller electrode 20_(U)with the plate-like workpiece 12_(H) is equal to that of the rollerelectrode 20_(S) with the plate-like workpiece 12_(M). Although theconventional apparatus permits the weld nugget N to be formed at aposition intermediate between the pair of roller electrodes 20_(U),20_(S), the position of the interface G of the two plate-like workpieces12_(H), 12_(M) is offset from the weld weld nugget N toward the thinplate-like workpiece 12_(M), as indicated in FIG. 11, whereby the nuggetratio R_(N) is as low as 20%, making it impossible to provide asufficiently large weld strength.

There will next be described other embodiments of this invention. In thefollowing embodiments, the reference numerals used in the precedingembodiment will be used to identify the corresponding elements, and nodescription of these elements will be provided.

FIGS. 12 and 13 show the pair of plate-like workpieces 12_(H), 12_(M)which are guided by guiding devices (guide rollers 14 and guide members18_(U), 18_(S)) and squeezed by the roller electrodes 20_(U), 20_(S), inother embodiments of this invention.

In the embodiment of FIG. 12, the pair of roller electrodes 20_(U),20_(S) have the respective outer circumferential contact surfaces28_(U), 28_(S) which have the same width dimension. However, the rollerelectrodes 20_(U), 20_(S) are disposed such that the widthwisecenterlines CL_(U), CL_(S) of the outer circumferential contact surfaces28_(U), 28_(S) are offset from each other by a predetermined distance L₂in the axial direction. Further, the guiding device (guide rollers 14and guide members 18_(U), 18_(S)) is constructed so as to guide the pairof plate-like workpieces 12_(H), 12_(M) such that the centerline of thelap width S of the overlapping edge portion of the workpieces 12_(H),12_(M), that is, the welding centerline A is substantially aligned withthe widthwise centerline CL_(S) of the roller electrode 20_(S) locatedon the side of the thin plate-like workpiece 12_(M).

In the embodiment of FIG. 13, the the outer circumferential surface28_(U) of the roller electrode 20_(U) on the side of the thickplate-like workpiece 12_(H) has a smaller width dimension than the outercircumferential contact surface 28_(S) of the roller electrode 20_(S) onthe side of the thin plate-like workpiece 12_(M). The pair of rollerelectrodes 20_(U), 20_(S) are disposed such that the widthwisecenterlines CL_(U), CL_(S) of those outer circumferential contactsurfaces 28_(U), 28_(S) are substantially aligned with each other in theaxial direction. Further, the guiding device (guide rollers 14 and guidemembers 18_(U), 18_(S)) is constructed so as to guide the two plate-likeworkpieces 12_(H), 12_(M) such that the centerline of the lap width S ofthe overlapping edge portion of the workpieces 12_(H), 12_(M), that is,the welding centerline A is substantially aligned with the widthwisecenterlines CL_(U), CL_(S) of the roller electrodes 20_(S), 20_(S).

In both of the embodiments of FIGS. 12 and 13, the surface area ofcontact of the roller electrode 20_(U) with the thick plate-likeworkpiece 12_(H) is made smaller than that of the roller electrode20_(S) with the thin plate-like workpiece 12_(M). When the pair ofplate-like .[.rollers.]. .Iadd.workpieces .Iaddend.12_(H), .[.12_(S) .]..Iadd.12_(M) .Iaddend.are squeezed by the pair of roller electrodes20_(U), 20_(S), the amount of mashing of the thick plate-like workpiece12_(H) by the roller electrode 20_(U) is made larger than the amount ofmashing of the thin plate-like workpiece 12_(M) by the roller electrode20_(S). Therefore, the interface G of the two plate-like workpieces12_(H) .[.,.]. .Iadd.and .Iaddend.12_(M) is formed at a positionintermediate between the pair of roller electrodes, and the nugget ratioR_(N) is increased, making it possible to provide a sufficient weldstrength.

FIG. 14 shows the pair of plate-like workpieces 12_(H), 12_(S) which areguided by guiding devices (guide rollers 14 and guide members 18_(U),18_(S)) and squeezed by the roller electrodes 20_(U), 20_(S), in anotherembodiment of this invention. In the embodiment shown in the figure, theouter circumferential surfaces 28_(U), 28_(S) of the pair of rollerelectrodes 20U, 20S have the same width dimension. However, the rollerelectrode 20_(U) located on the side of the thick plate-like workpiece20_(H) has a smaller diameter than the roller electrode 20_(S) locatedon the side of the thin plate-like workpiece 20_(M). This arrangement isadvantageous in that the amount of mashing of the thick plate-likeworkpiece 12_(H) is made larger than that of the thin plate-likeworkpiece 12_(M), even though the lengths W_(H) and W_(M) of contact ofthe pair of roller electrodes 20_(U), 20_(S) with the plate-likeworkpieces 12_(H), 12_(M) in the axial direction of the rollerelectrodes are equal to each other.

FIG. 15 shows the pair of plate-like workpieces 12_(H), 12_(S) which areguided by guiding devices (guide rollers 14 and guide members 18_(U),18_(S)) and squeezed by the roller electrodes 20_(U), 20_(S), in anotherembodiment of this invention. In the embodiment shown in the figure, theouter circumferential contact surfaces 28_(U), 28_(S) of the pair ofroller electrodes 20_(U), 20_(S) have the same width dimension. Theseroller electrodes 20_(U), 20_(S) are disposed such that the widthwisecenterlines CL_(U), CL_(S) of the outer circumferential contact surfaces28_(U), 28_(S) are substantially aligned with each other. Theabove-indicated guiding device (guide rollers 14 and guide members18_(U), 18_(S)) is constructed to guide the pair of plate-likeworkpieces 12_(H), 12_(M) such that the centerline of the lap width S ofthe overlapping edge portions of the pair of plate-like workpieces12_(H), 12_(M), that is, the welding centerline A is substantiallyaligned with the widthwise centerlines CL_(U), CL_(S) of the pair ofroller electrodes 20_(U), 20_(S). In the present embodiment, the drivemotors 76 functioning as a driving device for driving the guide rollers14 and retainer rollers 16 are arranged to rotate the guide rollers 14and retainer rollers 16 so that the welding speed, namely, theperipheral speed of the roller electrodes 20_(U), 20_(S) is 20 m/min. orhigher.

In the present embodiment, the weld nugget N is formed obliquely alongthe interface G, as indicated in FIG. 16 by way of example. Accordingly,the nugget ratio R_(N) is made large permitting a sufficient weldstrength, even though the amount of mashing of the thick plate-likeworkpiece 12_(H) is not made larger than that of the thin plate-likeworkpiece 12_(M). This appears to be caused by the specific resistanceof the plate-like workpieces 12_(H), 12_(M) which becomes higher thanelectrical contact resistance between the pair of roller electrodes20_(U), 20_(S) and the plate-like workpieces 12_(H), 12_(M).

FIG. 17 shows a relationship between the thickness ratio of theplate-like workpieces 12_(H), 12_(M) and the welding speed, which wasobtained by experiments conducted by the present inventors in differentconditions, each experiment consisting of ten test operations. In thefigure, (1) indicates the case where the nugget ratio R_(N) is lowerthan a desired value (40%), while (2) indicates the case where thenugget ratio R_(N) is higher than the desired value (40%). It will beunderstood from the table of FIG. 17 that the nugget ratio R_(N) doesnot reach the desired value even where the thickness ratio is 1.4(1:1.4), when the welding speed is 10 m/min. or 15 m/min., while thenugget ratio R_(N) reaches the desired value even where the thicknessratio is 2.0 (1.2.0), when the welding speed is 20 m/min.

FIG. 18 shows the state of a weld when the welding speed is 20 m/min. orhigher in the embodiment of FIGS. 1-9. In this case, the interface G isformed at a position intermediate between the pair of roller electrodes20_(U), 20_(S), and the nugget N is formed obliquely along the interfaceG, whereby the nugget ratio R_(N) is further increased, leading to anadvantage of assuring a sufficient weld strength.

FIG. 19 shows an embodiment provided with a preliminary treatment devicefor performing a preliminary treatment step of mashing and thinning awelding portion or one edge portion of the thick workpiece 12_(H) priorto a welding operation. In the embodiment of FIG. 19, the pair ofplate-like workpieces 12_(H), 12_(M) are guided relative to each other(Guiding Step) by a plurality of sets of guide rollers 14 and guidemembers 18_(U), 18_(S) functioning as a guiding device, in the samemanner as in the preceding embodiments, such that the edge portions ofthe workpieces to be welded together overlap each other with apredetermined lap amount S, while the workpieces are fed in thepredetermined feeding direction B by conveyors 140, 142, 144, guiderollers 14 and retainer rollers 16, which function as a work feedingdevice. Then, the overlapping edge portions of the thus positioned pairof plate-like workpieces 12_(H), 12_(M) are squeezed at the weldingportion by the pair of upper and lower roller electrodes 20_(U), 20_(S),and are seam-welded (Welding Step) by application of a welding currentbetween the roller electrodes 20_(U), 20_(S), along the weldingcenterline A which is parallel to the feeding direction B and whichpasses the widthwise center of the lap width S of the overlapping edgeportions. The pairs of plate-like workpieces 12_(H), 12_(M) which havebeen welded together are unloaded by a conveyor 150 and placed in stackon a pallet 152.

In a feeding path along which the workpieces are fed as described above,there is provided a presser roller device 148 consisting of a pair ofroller dies 146_(U), 146_(S) for squeezing and thinning the weldingportion or edge portion of the thick plate-like workpiece 12_(H). Thepresser roller device 148 is disposed upstream of the roller electrodes20_(U), 20_(S), guide rollers 14 and guide members 18_(U), 18_(S), thatis, between the conveyors 140 and 142 provided exclusively for feedingthe thick plate-like workpiece 12_(H). This presser roller device 148functions as the preliminary treatment device for thinning the edgeportion of the thick plate-like workpiece 12_(H) prior to the weldingoperation. In the mash seam welding apparatus according to the presentembodiment, the thickness of the welding portion of the thick plate-likeworkpiece 12_(H) is reduced prior to the seam welding by the pair ofroller electrodes 20_(U), 20_(S), so that the thickness difference ofthe welding portion of the thick plate-like workpiece 12_(H) withrespect to the welding portion of the thin plate-like workpiece 12_(M)is reduced, whereby the weld nugget N formed at a position intermediatebetween the pair of roller electrodes 20_(U), 20_(S) extends across theinterface G of the two plate-like workpieces 12_(H), 12_(M), permittingthe weld to have a desired strength.

While the embodiments of this invention have been explained referring tothe drawings, the invention may be otherwise embodied.

In the illustrated embodiments, the plate-like workpiece 12_(H) has alarger thickness than the plate-like workpiece 12_(M), the workpiece12_(M) may have a thickness larger than the workpiece 12_(H). In thiscase, too, the surface area of contact of the thick plate-like workpiecewith the corresponding roller electrode is made larger than that of thethin plate-like workpiece with the corresponding roller electrode.

While the illustrated embodiments are adapted to seam-weld theplate-like workpieces 12_(H), 12_(M) at their edge portions, theplate-like workpieces may be seam-welded at their portions locatedinwardly from the edge portions.

The embodiment of FIG. 18 is a combination of the embodiment of FIGS.1-9 and the embodiment of FIG. 16. However, the present invention may beembodied as any combination of the embodiments selected from among theembodiment of FIGS. 1-9, embodiment of FIG. 12, embodiment of FIG. 13,embodiment of FIG. 16 and embodiment of FIG. 19.

While the illustrated embodiments are adapted to seam weld the pair ofplate-like workpieces 12_(H), 12_(M) while the workpieces are fed in thehorizontal direction, the seam welding may be effected while theworkpieces are fed in the vertical direction.

Although the embodiment of FIG. 19 uses the presser roller device 148provided with the pair of roller dies 146_(U), 146_(S) as thepreliminary treatment device for thinning the edge portion of the thickplate-like workpiece 12_(H), the preliminary treatment device may be agrinding device for thinning the edge portion of the thick plate-likeworkpiece 12_(H) by grinding prior to the welding operation.

In the illustrated embodiments, the guide rollers 14 and the retainerrollers 16 include the lower guide rollers 14_(HS), 14_(MS) and thelower retainer rollers 16_(HS), 16_(MS) which are located below theplate-like workpieces 12_(H), 12_(M) and which are driven by the drivemotors 76. However, the upper guide rollers 14_(HU), 14_(MU) and theupper retainer rollers 16_(HU), 16_(MU) are driven by the drive motors76, or the upper and lower guide and retainer rollers are both driven bythe drive motors 76.

While all of the guide rollers 14_(HS), 14_(MS) are driven by the drivemotors 76 in the illustrated embodiments, only the selected ones ofthese guide rollers may be driven. In the case where the pair ofplate-like workpieces 12_(H), 12_(M) can be sufficiently fed by aplurality of sets of guide rollers 14, the set of retainer rollers 16 inthe embodiment of FIGS. 1-9 need not be driven.

Further, at least one of the pair of roller electrodes 20_(U), 20_(S) inthe embodiment of FIGS. 1-9 may be driven by the electric motor 140shown in FIG. 19, for example, at a peripheral speed equal to thefeeding speed of the pair of plate-like workpieces 12_(H), 12_(M).

Further, at least one of the pair of roller electrodes 20_(U), 20_(S) inthe embodiment of FIGS. 1-9 may be driven by a drive motor (not shown)at a peripheral speed equal to or higher than the feeding speed of thepair of plate-like workpieces 12_(H), 12_(M).

In the embodiment of FIGS. 1-9, the guide rollers 14 are inclined by apredetermined angle θ for pressing the pair of plate-like workpieces12_(H), 12_(M) toward the guide members 18_(U), 18_(S) while theworkpieces are fed. However, slant guiding plates may be provided forcontact with the outer edges of the pair of plate-like workpieces12_(H), 12_(M), such that the slant guiding plates approach the guidemembers 18_(U), 18_(S) as it extends in the feeding direction.

It is to be understood that the foregoing embodiments are provided forillustrative purpose only, and that various changes may be made in theillustrated embodiments, without departing from the spirit of theinvention.

We claim:
 1. A mash seam welding process of continuously welding a firstplate-like workpiece and a second plate-like workpiece having a smallerthickness than said first plate-like workpiece, at overlapping portionsof .Iadd.said .Iaddend.first and second plate-like workpieces, byapplying a welding current between a pair of roller electrodes disposedrotatably about respective axes thereof, to thereby form a .[.weld.]..Iadd.welding .Iaddend.nugget at an interface of said overlappingportions while said overlapping portions are squeezed by said pair ofroller electrodes, wherein .Iadd.a surface area of contact of one ofsaid pair of roller electrodes with said first plate-like workpiece ismade smaller than a surface area of contact of the other of said pair ofroller electrodes with said second plate-like workpieces, so that.Iaddend.an amount of mashing of said first plate-like workpiece by.Iadd.said .Iaddend.one .[.of said pair of.]. roller .[.electrodes.]..Iadd.electrode .Iaddend.is made larger than that of said secondplate-like workpiece by .[.the.]. .Iadd.said .Iaddend.other .[.of saidpair of.]. roller .[.electrodes.]. .Iadd.electrode.Iaddend.. .[.2. Amash seam welding process according to claim 1 wherein a surface area ofcontact of said one roller electrode with said first plate-likeworkpiece is made smaller than a surface area of contact of said otherroller electrode with said second plate-like workpiece..].3. A mash seamwelding process according to claim 1, wherein a length of contact ofsaid one roller electrode with said first plate-like workpiece in anaxial direction of said pair of roller electrodes is made smaller thanthat of said other roller electrode with said second plate-likeworkpiece in said axial direction.
 4. A mash seam welding processaccording to claim 1, wherein said one roller electrode located on theside of said first plate-like workpiece has a smaller diameter than saidother roller electrode located on the side of said second plate-likeworkpiece. .[.5. A mash seam welding process of continuously welding afirst plate-like workpiece and a second plate-like workpiece having asmaller thickness than said first plate-like workpiece, at overlappingportions of said first and second plate-like workpieces, by applying awelding current between a pair of roller electrodes disposed rotatablyabout respective axes thereof, to thereby form a weld nugget at aninterface of said overlapping portions while said overlapping portionsare squeezed by said pair of roller electrodes, said processcomprising:feeding a pair of plate-like workpieces at a speed of 20m/min. or higher while said plate-like workpieces are squeezed by saidpair of roller electrodes..]..[.6. A mash seam welding process ofcontinuously welding a first plate-like workpiece and a secondplate-like workpiece having a smaller thickness than said firstplate-like workpiece, at overlapping portions of said first and secondplate-like workpieces, while said overlapping portions of said first andsecond plate-like workpieces are squeezed by a pair of roller electrodesdisposed rotatably about respective two parallel axes thereof, saidprocess comprising: a welding step of continuously welding said firstand second plate-like workpieces by applying a welding current betweensaid pair of roller electrodes, to thereby form a weld nugget at aninterface of said overlapping portions while said overlapping portionsare squeezed by said pair of roller electrodes; and a preliminarytreatment step of squeezing a welding portion of said first plate-likeworkpiece between upper and lower roller dies of a presser roller devicethereby to reduce the thickness of said welding portion of said firstplate-like workpiece, prior to said welding step..].7. A mash seamwelding apparatus for continuously welding a first plate-like workpieceand a second plate-like workpiece having a smaller thickness than saidfirst plate-like workpiece, at overlapping portions of said first andsecond plate-like workpieces, by applying a welding current between apair of roller electrodes disposed rotatably about respective axesthereof, to thereby form a weld nugget at an interface of saidoverlapping portions while said overlapping portions are squeezed bysaid pair of roller electrodes, said mash seam welding apparatuscomprising:said pair of roller electrodes being biased toward each otherand having a same diameter; and a guiding device for guiding said firstand second plate-like workpieces such that said two plate-likeworkpieces overlap each other with a predetermined lap amount, whilesaid first and second plate-like workpieces are fed toward said pair ofroller electrodes, so that said overlapping portions of said first andsecond plate-like workpieces are squeezed by said pair of rollerelectrodes such that a surface area of contact of one of said pair ofroller electrodes with said first plate-like workpiece is smaller than asurface area of contact of the other of said pair of roller electrodeswith said second plate-like workpiece.
 8. A mash seam welding.[.process.]. .Iadd.apparatus .Iaddend.according to claim 7, whereinsaid guiding device guides said first and second plate-like workpiecessuch that a length of contact of said one roller electrode with saidfirst plate-like workpiece in an axial direction of said pair of rollerelectrodes is smaller than that of said other roller electrode with saidsecond plate-like workpiece in said axial direction.
 9. A mash seamwelding .[.process.]. .Iadd.apparatus .Iaddend.according to claim 8,wherein said pair of roller electrodes have respective outercircumferential contact surfaces and are disposed such that widthwisecenterlines of said outer circumferential contact .[.surf aces.]..Iadd.surfaces .Iaddend.are substantially aligned with each other, andwherein said guiding device guides said first and second plate-likeworkpieces such that an edge of said first plate-like workpiece islocated between a widthwise end of said outer circumferential contactsurface of said one roller electrode on the side of said firstplate-like workpiece and said widthwise centerline of said outercircumferential contact surface of said one roller electrode, while anedge of said second plate-like workpiece is located at an widthwise endof said outer circumferential contact surface of said other rollerelectrode on the side of said first plate-like workpiece.
 10. A mashseam welding apparatus according to claim 8, wherein said pair of rollerelectrodes have respective outer circumferential contact surfaces,respectively, and are disposed such that widthwise centerlines of saidouter circumferential contact surfaces are offset from each other by apredetermined distance in an axial direction of said pair of rollerelectrodes, and wherein said guiding device guides said first and secondplate-like workpieces such that a centerline of overlapping edgeportions of said first and second plate-like workpieces is substantiallyaligned with said widthwise centerline of said outer circumferentialcontact surface of said one roller electrode.
 11. A mash seam weldingapparatus according to claim 8, wherein said pair of roller electrodeshave respective outer circumferential contact surfaces, respectively,said outer circumferential contact surface of said one roller electrodelocated on the side of said first plate-like workpiece having a smallerwidth dimension than that of said outer circumferential contact surfaceof said other roller electrode located on the side of said secondplate-like workpiece, said pair of roller electrodes being disposed suchthat widthwise centerlines of said outer circumferential contactsurfaces are substantially aligned with each other, and wherein saidguiding device guides said first and second plate-like workpieces suchthat a centerline of overlapping edge portions of said first and secondplate-like workpieces is substantially aligned with said widthwisecenterlines of said outer circumferential contact surfaces of said pairof roller electrodes. .[.12. A mash seam welding apparatus according toclaim 7, wherein said one roller electrode located on the side of saidfirst plate-like workpiece has a smaller diameter than said other rollerelectrode located on the side of said second plate-likeworkpiece..]..[.13. A mash seam welding apparatus according to claim 7,further comprising a feeding device for feeding said first and secondplate-like workpieces at a speed of 20 m/min. or higher while said firstand second plate-like workpieces are squeezed by said first and secondroller electrodes..].14. A mash seam welding apparatus for continuouslywelding a first plate-like workpiece .[.(12_(H)).]. and a secondplate-like workpiece .[.(12_(M)).]. having a smaller thickness than saidfirst plate-like workpiece, at overlapping portions of said first andsecond plate-like workpieces, by applying a welding current between apair of roller electrodes .[.(20).]. disposed rotatably about respectiveaxes .[.(C3).]. thereof, to thereby form a weld nugget .[.(N).]. at aninterface .[.(G).]. of said overlapping portions while said overlappingportions are squeezed by said pair of roller electrodes, said mash seamwelding apparatus comprising:said pair of roller electrodes being biasedtoward each other and having a same diameter; a guiding device forguiding said first and second plate-like workpieces such that said firstand second plate-like workpieces overlap each other with a predeterminedlap amount, while said first and second plate-like workpieces are fedtoward said pair of roller electrodes, so that said overlapping portionsof said first and second plate-like workpieces are squeezed by said pairof roller electrodes such that .Iadd.a surface area of contact of one ofsaid pair of roller electrodes with said first plate-like workpiece ismade smaller than a surface area of contact of the other of said pair ofroller electrodes with said second plate-like workpiece, so that.Iaddend.an amount of mashing of said first plate-like workpiece by.Iadd.said .Iaddend.one .[.of said pair of.]. roller .[.electrodes.]..Iadd.electrode .Iaddend.is made larger than that of said secondplate-like workpiece by .[.the.]. .Iadd.said .Iaddend.other .[.of saidpair of.]. roller .[.electrodes.]. .Iadd.electrode.Iaddend.; and afeeding device for feeding said first and second plate-like workpiecesat a speed of 20 m/min. or higher while said overlapping portions aresqueezed by said pair of roller electrodes. .[.15. A mash seam weldingapparatus for continuously welding a first plate-like workpiece and asecond plate-like workpiece having a smaller thickness than said firstplate-like workpiece, at overlapping portions of said first and secondplate-like workpieces, while said overlapping portions of said first andsecond plate-like workpieces are squeezed by a pair of roller electrodesdisposed rotatably about respective two parallel axes thereof, said mashseam welding apparatus comprising:a work feeding device for feeding saidfirst and second plate-like workpieces in a predetermined feedingdirection; a welding portion for continuously welding said first andsecond plate-like workpieces by applying a welding current between saidpair of roller electrodes, to thereby form a weld nugget at an interfaceof said overlapping portions, while said first and second plate-likeworkpieces are fed by said work feeding device and squeezed by said pairof roller electrodes; and a preliminary treatment device comprising apresser roller device including upper and lower dies disposed upstreamof said welding portion, for squeezing the portion of said firstplate-like workpiece which overlaps said second plate-like workpiece,between said upper and lower roller dies thereby to reduce the thicknessof said portion of said first plate-like workpiece..]..Iadd.16. A mashseam welding apparatus for continuously welding a first plate-likeworkpiece and a second plate-like workpiece having a smaller thicknessthan said first plate-like workpiece, at overlapping portions of saidfirst and second plate-like workpieces, by applying a welding currentbetween a pair of roller electrodes disposed rotatably about respectiveaxes thereof, to thereby form a weld nugget at an interface of saidoverlapping portions while said overlapping portions are squeezed bysaid pair of roller electrodes, said mash seam welding apparatuscomprising:said pair of roller electrodes being biased toward eachother, one of said pair of roller electrodes located on the side of saidfirst plate-like workpiece having a smaller diameter than that of theother of said pair of roller electrodes located on the side of saidsecond plate-like workpiece; and a guiding device for guiding said firstand second plate-like workpieces such that said two plate-likeworkpieces overlap each other by a predetermined lap amount, while saidtwo plate-like workpieces are fed toward said pair of roller electrodes,so that said overlapping portions of said first and second plate-likeworkpieces are squeezed by said pair of roller electrodes such that asurface area of contact of said one roller electrode with said firstplate-like workpiece is smaller than a surface area of contact of saidother roller electrode with said second plate-like workpiece..Iaddend.